1. Field of the Invention
The present invention relates to backlight devices for backside illumination of liquid crystal display device and, more specifically, to an edge-light type backlight device which introduces a light from a side surface of a light-guiding plate made of a transparent plate.
2. Description of the Related Art
A liquid crystal panel of a liquid crystal display device performs a screen display by not emitting a light by itself but transmitting and interrupting a light by a polarization action. Therefore, generally, a backlight device, which illuminates a liquid crystal panel from a backside, is incorporated in a liquid crystal display device.
FIG. 1 is an exploded perspective view of a conventional backlight device. The conventional backlight device shown in FIG. 1 comprises reflectors 12, in which fluorescent tubes 11 or the like as a light source are accommodated and held, and a light-guiding plate 13 made of a transparent plate. The reflector 12 are located along side surfaces of the light-guiding plate 13 so as to reflect a light emitted from the fluorescent tubes 11 and introduce the light to the light-guiding plate 13. Each reflector 12 is generally formed of a metal plate in a channel shape, and arranged so that an opening part faces the side surface of the light-guiding plate 13. The light-guiding plate 13 is formed of, for example, a highly transparent PMMA resin, and has a thickness of about 10 mm.
A diffusion plate 14 is arranged on a light-emitting surface (a surface on a side where the liquid crystal panel is arranged) of the light-guiding plate 13. The diffusion plate 14 is formed, for example, of a polyester base resin, and a thickness thereof is about 0.1 mm. A reflection plate 15 is arranged on a backside opposite to the light-emitting surface of the light-guiding plate 13. The reflection plate 15 is formed of, for example, a polyester base resin.
The reflectors 12, the light-guiding plate 13, the diffusion plate 14, and the reflection plate 15 are accommodated in a backside cover 16, which is formed of, for example, an aluminum plate having a thickness of 0.8 mm. A frame 17 formed of an aluminum plate is provided over the diffusion plate 14. It should be noted that the fluorescent tubes 11 as a light source are held in the reflectors 12 by holding members 18 made of a silicon rubber having a thickness of 1.0 mm.
FIG. 2 is a plan view of the backlight device shown in FIG. 1. The periphery of the backlight device is covered by the frame 17, and the diffusion plate 14 is exposed inside the frame 17. A light introduced from the fluorescent tubes 11 into the light-guiding plate 13 is emitted from the diffusion plate 14 at a uniform illumination intensity. The liquid crystal panel is illuminated from the backside by this light.
FIG. 3 is an enlarged cross-sectional view taken along a line III-III in FIG. 2. As shown in FIG. 3, the light-guiding plate 13 is accommodated in the backside cover 16. The reflector 12 is arranged along and near the side surface of the light-guiding plate 13. The light-guiding plate 13 and the reflectors 12 are held in a state where they are sandwiched between the frame 17 and the backside cover 16.
According to the above-mentioned arrangement structure, if an air gap between the light-guiding plate and each reflector 12 is reduced due to a thermal expansion of the light-guiding plate 13 and when a shock is applied to the backlight device, it is possible that the light-guiding plate 13 contacts with the fluorescent tubes 11 in the reflectors 12. In a worst case, the fluorescent tubes 11, which are made of glass tubes, may be damaged due to the contact with the light-guiding plate 13.
In order to prevent such a problem, it is considered to form a structure in which an opening part of each reflector 12 facing the light-guiding plate 13 is made smaller than the side surface of the light-guiding plate 13. That is, a size of the opening part of the reflector 12 is set so that the light-guiding plate 13 does not contact with the fluorescent tubes 11 by bringing the light-guiding plate 13 into contact with a rim of the reflector 12.
Thus, there is suggested a structure to fix the reflector itself relative to the light-guiding plate. That is, a groove is formed on a surface of the light-guiding plate near the side surface thereof so as to couple the light-guiding plate and the reflector by bringing the rim of the reflector into engagement with the groove so as to maintain the positional relationship between the light-guiding plate and the reflector always constant (for example, refer to Patent Document 1). Alternatively, a step is made on an end part of the reflector facing the light-guiding plate so as to form a structure in which the reflector covers the light-guiding plate over the portion of the step.
Patent Document 1: Japanese Laid-Open Patent Application No. 7-287228
Patent Document 2: Japanese Laid-Open Patent Application No. 9-189911
As mentioned above, according to the structure which maintains a distance between the reflector and the light-guiding plate constant by bringing the reflector and the light-guiding plate into engagement with each other, there is no possibility that the light-guiding plate contacts with the fluorescent tubes in the reflector, which can prevent the fluorescent tubes from damaging.
However, if such a structure is made, an area of the opening part, through which the light introduced from the reflector into the light-guiding plate passes, is reduced, which may generate a problem that a luminance is greatly reduced. That is, in the structure disclosed in Patent Document 1, since the groove is formed on the light-guiding plate so as to insert a rim of the reflector into the groove, an area of the light-guiding plate through which a light passes is reduced due to the groove, which results in a reduction in the luminance. Additionally, also in the structure disclosed in Patent Document D2, the area of the opening part is reduced by an area corresponding to the steps provided on the end part of the reflector, which results in a reduction in the luminance.